An automatic firearm, which has been fired repeatedly and continously, may have the temperature of its firing chamber raised to a point which could engender cook-off of a cartridge disposed therein. Cook-off is that phenomenon where a chambered cartridge fires without being struck by the firing pin of the firearm. The prior art has recognized the aforementioned problem and in response thereto has provided open bolt firing mechanisms which hold the breech bolt in retracted position after automatic firing, thereby preventing a live round from remaining in the hot firing chamber. Examples of open bolt firing mechanisms may be found in U.S. Pat. Nos. 2,383,487; 3,618,455; 3,960,053 and 4,057,003. State of the art open bolt firing mechanisms, however, are not of simple construction and embody a relatively large number of component parts.